linux-kernel - Re: [PATCH v3 04/10] dt-bindings: iio: dac: ad3552r: add io-backend support

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Message-ID: <7db42c1b080ea98c0b02ad91ffb1436c9b9218ff.camel@gmail.com>
Date: Mon, 30 Sep 2024 10:24:21 +0200
From: Nuno Sá <noname.nuno@...il.com>
To: Krzysztof Kozlowski <krzk@...nel.org>, Jonathan Cameron
 <jic23@...nel.org>
Cc: Angelo Dureghello <adureghello@...libre.com>, Lars-Peter Clausen	
 <lars@...afoo.de>, Michael Hennerich <Michael.Hennerich@...log.com>, Nuno
 Sa	 <nuno.sa@...log.com>, Rob Herring <robh@...nel.org>, Krzysztof
 Kozlowski	 <krzk+dt@...nel.org>, Conor Dooley <conor+dt@...nel.org>,
 Olivier Moysan	 <olivier.moysan@...s.st.com>, linux-iio@...r.kernel.org, 
	linux-kernel@...r.kernel.org, devicetree@...r.kernel.org,
 dlechner@...libre.com
Subject: Re: [PATCH v3 04/10] dt-bindings: iio: dac: ad3552r: add io-backend
 support

On Mon, 2024-09-30 at 09:31 +0200, Krzysztof Kozlowski wrote:
> On 30/09/2024 09:20, Nuno Sá wrote:
> > > > > > 
> > > > > > You described the driver, so how does it matter? Driver can call
> > > > > > get_backend_from_parent(), right? Or
> > > > > > get_backend_from_fwnode(parent)?  
> > > > > 
> > > > > Well yes, just stating what the framework (also in terms of bindings)
> > > > > is
> > > > > expecting. Of course that on the driver side we can paper around it
> > > > > the
> > > > > way we
> > > > > want. But my main point was that we can only paper around it if we use
> > > > > code that
> > > > > is meant not to be used.
> > > > > 
> > > > > And, FWIW, I was (trying) replying to your comment
> > > > > 
> > > > > "You can take it from the child-parent relationship"
> > > > > 
> > > > > Again, we can only do that by introducing new code or use code that's
> > > > > not
> > > > > meant
> > > > > to be used. The way we're supposed to reference backends is by
> > > > > explicitly
> > > > > using
> > > > > the proper FW property.
> > > > > 
> > > > > Put it in another way and a completely hypothetical case. If we have a
> > > > > spi
> > > > > controller which happens to export some clock and one of it's
> > > > > peripherals
> > > > > ends
> > > > > up using that clock, wouldn't we still use 'clocks' to reference that
> > > > > clock?  
> > > > 
> > > > I asked how coupled are these devices. Never got the answer and you are
> > > > reflecting with question. Depends. Please do not create hypothetical,
> > > > generic scenarios and then apply them to your one particular opposite
> > > > case.
> > > 
> > > I'll throw a possible clarifying question in here.  Could we use this
> > > device with a multimaster SPI setup such that the control is on a
> > > conventional
> > > SPI controller (maybe a qspi capable one), and the data plane only goes
> > > through
> > > a specific purpose backend?  If so, then they are not tightly coupled and
> > > the reference makes sense.  Putting it another way, the difference between
> > > this case and all the prior iio-backend bindings is the control and
> > > dataplanes
> > > use the same pins.  Does that have to be the case at the host end?  If it
> > > does,
> > > then the reference isn't strictly needed and this becomes a bit like
> > > registering a single device on an spi bus or an i2c bus depending on who
> > > does the registering (which is down to the parent in DT).
> > > 
> > 
> > So, we currently have two drivers (with a new one being added in this
> > series)
> > for the same device:
> > 
> > 1) A SPI one tied to a typical spi controller. This is the "low speed"
> > implementation and does not use backends;
> > 2) The new platform device that is connected like this to the backend.
> 
> Drivers, platform devices are Linux specifics. These were not our
> questions here. You are responding with description matching current
> Linux code.
> 
> > 
> > So yes, my understanding (but Angelo should know better :)) is that they are
> > tightly coupled. Putting it in another way, the new platform device is very
> > much
> > specific to this parent (and yeah, this is a very special usecase where
> > control
> 
> Again, Linux stuff.
> 
> > and data planes are controlled by the IIO backend) and should not exist with
> > it.
> 
> I pointed this issue already in this thread. You keep describing
> drivers, so of course they will be coupled as much as you write them.
> 

Well, because this is how it's being used and it's easy for me to fall into the
implementation but ok, I get your point. Directly then replying in terms of HW,
this could be used in a way where we have a typical spi controller handling the
device and the data plane only going through the backend. In fact, the HW folks
first tried the SPI ENGINE IP (which is a typically controller) but could not
get the maximum sampling rate out of the device so they came up with this custom
design. So, in theory is possible, in practise will likely never happen but I
guess that does not matter for the bindings?

- Nuno Sá